1. Field of the Invention
The present invention relates to a high-frequency module, and more particularly, to a resonator, a filter, an oscillator, or the like, for use in microwave band or millimeter wave band communication.
2. Description of Related Art
With the recently expanding demand for a mobile communication systems, the millimeter wave band has been widely used in order to increase the available information-carrying capacity.
For use in such a system, a high-frequency dielectric filter or a high-frequency voltage-controlled oscillator (VCO) may include, for example, a TE01xcex4-mode dielectric resonator in a cylindrical form.
In general, the resonant frequency of the resonator is determined by its shape, while the strength of electromagnetic coupling between the resonator and a micro-strip or the like is determined by the distance between them. Thus, in order to produce a filter or a resonator which meets a desired specification, the formation of a shape of the resonator and positioning of the same require great precision.
In Laid-open Japanese Patent Application No. 8-265015, the assignee of the present application has presented a module in which electrodes are arranged on both main surfaces of a dielectric sheet to form a dielectric resonator on a part of the sheet. The electrodes arranged on the dielectric sheet serve as ground potentials; and a microstrip arranged on another dielectric sheet is stacked on the dielectric sheet. This arrangement is used in a high-frequency module such as a VCO.
In addition, a similar type of high-frequency module has been presented in Japanese Patent Application Serial No. 8-294087 and the U.S. patent application Ser. No. 08/965,464, now U.S. Pat. No. 6,016,090. FIGS. 5 to 8 illustrate the structure of the high-frequency module. It should be noted that this high-frequency module was not laid-open to the public at the time of filing of the Japanese Application No. 10-17006 on which the present application is based. Thus, the inventors do not deem the high-frequency module of FIGS. 5-8 to be prior art with respect to the present invention.
FIG. 5 shows a perspective view of the module. Here, electrodes 2 and 3 are formed on both main surfaces of a dielectric sheet 1, excluding a part of each main surface of the sheet, where openings 4 and 5, respectively, are located. This permits a TE010-mode dielectric resonator to be formed. Two coupling lines 11 and 12, which are formed on a dielectric or insulating substrate 6, are magnetically coupled to the dielectric resonator.
As shown in FIG. 6, since the electromagnetic field is trapped between the openings 4 and 5, coupling between the various components, except between the lines 11 and 12 and the resonator, can be reduced. Moreover, the concentration of electromagnetic-field energy at the openings permits a strong coupling between the resonator and the coupling lines, resulting in an increase in the frequency range over which the oscillation frequency can vary, when the resonator is used as an oscillator.
However, the impedance of the lines 11 and 12 at the opening is a little higher than that of the lines in other areas, since no ground electrode is present at the upper and lower part of the opening. As a result, reflection of a transmitted signal due to impedance mismatching occurs, leading to a generation of a spurious resonance due to the electrical length between the part that causes the reflection and a negative resistance circuit. This is a problem which is unique to a TE010-mode dielectric resonator and is not a problem in a TE01xcex4-mode dielectric resonator.
In FIG. 5, the lines 11 and 12 passing over the opening are straight. With this shape, the impedance of each line increases with the distance away from the edge of the opening, i.e., toward the center of the opening, as shown in FIG. 7. That is, an impedance mismatch occurs. Thus, a resonance is produced at a frequency that is different from the desired resonant frequency of the dielectric resonator.
FIG. 8 shows a Smith chart in which the r direction indicates the magnitude of the reflection generated by a resonance, while the xcex8 direction denotes the phase of the reflection. In the module shown in FIG. 5, a resonance due to impedance mismatching of the coupling lines occurs, in tune with the spurious resonance produced by the dielectric resonator.
The module is designed under the assumption that the characteristic impedance of each coupling line is uniform. However, in practice, the characteristic impedance is position-dependent. Therefore, when the frequency of a signal applied to the module is changed, the distribution of the electromagnetic field around each coupling line is also changed. Thus, the coupling strength deviates from its expected value.
Thus, when such a resonator is used in a voltage-controlled oscillator, the position-dependence of the characteristic impedance of the coupling lines causes their characteristic impedance to change in response to a change in the oscillation frequency. Consequently, the linearity of frequency modulation with such a VCO becomes a problem to be considered.
Accordingly, it is desired to provide a high-frequency module that reduces parasitic oscillation due to impedance mismatching.
It is also desired to provide a high-frequency module which allows frequency modulation with enhanced linearity.
In the following summary of the invention, the symbol Z0 denotes a characteristic impedance of the area in which the ground electrode and the line are opposing, while the symbol Z1 denotes a characteristic impedance of the area in which they are not opposing, namely, the area over the opening. When an end of the line is resistively terminated, the magnitude of a reflection coefficient at the point where the line is electro-magnetically coupled to a dielectric resonator (hereinafter referred to as a resonant point) is represented by the formula: (Z12xe2x88x92Z02)/(Z12+Z02). The larger the ratio of Z1/Z0, the greater the reflection.
On the other hand, a capacitive component of the line exists mainly at a location where the edge of the opening opposes the line. Thus, by shortening the distance between the edge of the opening and the line, the capacitance of the line can be increased, and the characteristic impedance of the line can be suppressed. In addition, reducing the position-dependency of the characteristic impedance of the line permits the linearity of frequency modulation to be enhanced.
This invention provides a high-frequency module including a dielectric sheet, electrodes disposed on both main surfaces of the sheet, an opening formed in each electrode and aligned so that the openings form a dielectric resonator, lines coupled to the dielectric resonator, a substrate having the lines thereon and being stacked on the dielectric sheet, and a conductive case to contain the substrate and the dielectric sheet; in which a first one of the lines coupled to the dielectric resonator is arranged along a path corresponding to the inside of the opening and running substantially along the edge of the opening.
With the above arrangement, the impedance of the line around the opening is not increased, leading to a reduction in the amount of reflected RF energy. There is also a second line, one end of which may be resistively terminated and the other end of which may be connected to a negative resistance circuit so as to form an oscillator. Thus, a parasitic oscillation can be controlled.
Furthermore, an end of the first line coupled to the dielectric resonator may be connected to a variable reactive element. Moreover, one end of the second line may be resistively terminated and the other end of the second line may be connected to a negative reactive circuit to form an oscillator. This arrangement permits the linearity of frequency modulation to be enhanced, since the position-dependency of the characteristic impedance of the line connected to the variable reactive element is small.
Moreover, if desired, both of the lines may be arranged at positions which correspond to the inside of the opening and run substantially along the edge of the opening. This permits parasitic oscillation to be suppressed, so that an oscillator having a high linearity of frequency modulation can be obtained.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.